1. Field of the Invention
The present invention is generally related to a method for manufacturing semiconductor devices, and more particularly to a method for forming an oxide protective film on the bonding pads of a semiconductor chip, using excited oxygen generated from air or O.sub.3 by UV irradiation.
2. Prior Art
Semiconductor chips, each formed with a desired circuit pattern by a wafer process, are subjected to a bonding pad-mounting process for facilitating making electrical connections to external elements. After depositing an Al-metal layer on a chip, the whole surface of the chip is covered with a passivation layer, except the bonding pad mounting areas, and then subjected to a packaging process. The purpose of the packaging process is to protect chip from the external environmental influences such as dust, heat, moisture, electrical and mechanical loads and the like, and to electrically connect the semiconductor elements to the external elements. In the packaging process, the chip is encapsulated with metal, ceramic material, or resin (a resin-type molding material) by an encapsulation process.
The package encapsulated with a plastic material, such as epoxy resin, has many advantages from economic and productivity viewpoints. Nevertheless, it also has a disadvantage, for example that the corrosion at aluminum bonding pads frequently occurs due to moisture absorbed by the epoxy resin and penetrated through the gap between the lead frame of the packaged chip, and the plastic resin, or package cracks. In particular, during Pressure Cooker Test (PCT), a method for testing the reliability of the packaged device in which the package is subjected to moisture environment at about 2 atm., at 121.+-.2.degree. C., moisture penetrates into the epoxy resin, or into the gap between the resin and the lead frame, which results in corrosion at the bonding pads where protection by a passivation layer is not provided.
Various attempts have been made to prevent this corrosion. For example, methods have been proposed, in which a protective layer, for preventing penetrations of moisture and halogen ions which cause corrosion, is formed on the bonding pads. However, these methods are not commonly used, since it is difficult to apply them to the packaging process, or to obtain satisfactory results.
In particular, JP 56-116634A discloses a method for forming a protective film on the aluminum bonding pads by exposing the semiconductor device to high temperature vapor, after conducting the wire-bonding step. Further, in JP 52-50687A, the semiconductor device is immersed in high temperature water, at 80.degree.-250.degree. C., for 5-100 minutes, to form a hydroxide film on the aluminum bonding pads. These methods, however, also cannot be applied in practical uses, since the vapor or water inherently contains OH.sup.- or H.sup.+ and these may cause corrosion.
Another method has been proposed, in which a polymer such as silicone gel is deposited onto the surface of the chip, after wire bonding process, to form a diffusion barrier film. Although the diffusion barrier film is formed to prevent penetration of moisture or ions, the film cannot provide effective protection from moisture or ions, because the polymer itself has a relatively high water-absorption properties.
Miyakawa proposed a method for forming a fine aluminum oxide film on aluminum bonding pads, in order to prevent corrosion due to moisture penetration. [Improvement of Moisture Resistance by the New Surface Treatment of Aluminum Bonding Pads in LSI, T. Miyakawa et al., ISTFA '93, The 19th International Symposium for Testing & Failure Analysis] Miyakawa teaches immersion of a semiconductor wafer in an O.sub.3 solution in the last step of the wafer production process, to form an aluminum oxide film over the bonding pads. By doing this, failures during the PCT can be significantly decreased.
However, according to the Miyakawa's method, the aluminum oxide film is formed by bubbling O.sub.3 into deionized water (DI water) to increase the concentration of O.sub.3 in the water, and then immersing the wafer thereinto. Therefore, the reactivity of O.sub.3 in water is lower than that of O.sub.3 in air, because the former has a lower diffusion coefficient than the latter. Further, the wafer is exposed to contaminants in the O.sub.3 solution.